1. Field of the Invention
The present invention relates to a time base control system in the case where information, such as a video signal, an audio signal or the like, is recorded onto or reproduced from a recording medium, such as a disk, a magnetic tape or the like. The invention also relates to a scanning system in an information reproducing apparatus.
2. Background of the Art
Recording disks include CAV (constant angular velocity) disks and CLV (constant linear velocity) disks. In a CAV disk, a fixed quantity of information, that is, for example, video information for one frame, is recorded in every track of 360.degree. rotation formed in every position in the radial direction of the disk. Portions of the tracks in which pieces of information are recorded respectively corresponding to vertical synchronizing signals are located at boundaries between frames and are aligned on a straight line extended in the radial direction of the disk. Accordingly, in a CAV disk, no disturbance occurs in a period of a synchronizing signal in a reproduced video signal just after a jump-over operation so that specific reproduction such as still picture reproduction, or the like, can be carried out well.
In the case of a CLV disk, however, the quantity of information recorded on every track of one rotation varies with the radial position of the track, such that, for example, video information for one frame is recorded in a track in the innermost circumference, and, for example, video information for three frames is recorded in a track in the outermost circumference. In a CLV disk, therefore, a disturbance may occur in the period of a synchronizing signal in a video signal obtained just after a jump operation. Accordingly, an information reproducing apparatus has been proposed in which a video memory having a storage capacity for storing a video signal for one frame or for one field is provided, a video signal is converted into a digital signal in accordance with a clock synchronized with a reproducing video signal, and the thus converted digital signal is written into a predetermined position of the video memory and read out in accordance with an external stable clock.
In such an information reproducing apparatus, it is possible to carry out high-speed reproduction by a scanning operation, that is, by alternately performing a play operation and a track jump operation of an information detecting point by a pickup. At that time, it is considered that the track jump-over operation is performed such that a slider servo for controlling the position of a slider for carrying the pickup in the radial direction of the recording disk is turned off so as to forcedly move the slider and, at the same time, a tracking servo loop is made open for a predetermined time interval. In that case, a quantity of deviation of a tracking actuator incorporated in the pickup is gradually increased so as to open the tracking servo loop so that the tracking actuator returns to the middle point of its movable region. The information detecting point is caused to jump by the operation of the tracking actuator. The contents stored in the video memory are rewritten in synchronism with the track jump movement so that the high-speed reproduction having no picture disturbance owing to the track jump-over movement can be carried out.
In such a conventional scanning system, however, the play operation is required to be continued for a long time (about 33 msec) so that video information of one field can be reliably obtained when the play operation is performed with the tracking servo loop closed. Therefore, there has been a possibility that the quantity of deviation of the tracking actuator becomes large to thereby increase the demands on the tracking actuator, and thus easily cause trouble. Further, the number of the tracks jumped over by the information detecting point of the pickup is increased in the opened state of the tracking servo loop so that the difference is frequency between the video signals obtained just before and just after the servo loop is opened becomes large resulting in an increase in burden to a time base servo. Accordingly, particularly in a disk having large eccentricity, a time base error may come out of a range of control by a CCD which removes the time base error, causing color disturbance.
One type of time base control system employed in video disk players has the double function of rough tuning and fine tuning of a time base. The rough tuning is carried out by controlling the rotational speed of a spindle motor for rotationally driving a recording disk so as to control the relative speed between the recording disk and an information-detection point of a pickup acting as a signal recording/reproducing means. The fine tuning of the time base is carried out by feeding the reproduced signal obtained by the pickup to a clock-response delay element such as a CCD (charge coupled device).
In such a conventional system, although the time base control can be reliably performed as the controllable range of the fine tuning is made wider, the size of the circuit becomes undesirably large if the number of stages of CCDs is increased in order to expand the controllable range of the fine tuning.